Demographic and competition studies on Brachionus ibericus and Proales similis in relation to salinity and algal (Nannochloropsis oculata) density

Abstract

We isolated the rotifers Brachionus ibericus and Proales similis from the sediment of shrimp tanks and studied their individual demographic characters and competition between them at two food levels (0.25 × 10⁶, 1.00 × 10⁶ cells ml⁻¹ of Nannochloropsis oculata at 25 °C) and salinities ranging from 10 to 30‰. Our hypothesis was that growth rates would be higher with increasing food levels and salinities. Observations were taken twice a day for life table studies and daily once for population growth experiments. Using survivorship and fecundity data, we derived various life history variables. Although the average life span (7.6 ± 0.4 days) and gross reproductive rate (33.8 ± 2.9 neonate female⁻¹ day⁻¹) of B. ibericus were higher than those of P. similis (average life span 5.4 ± 0.6 days and gross reproductive rate 13.0 ± 0.6 neonate female⁻¹ day⁻¹), the population growth experiments showed that P. similis had higher r values (0.32 ± 0.005 day⁻¹) than B. ibericus (0.23 ± 0.002 day⁻¹) at 1.0 × 10⁶ cells ml⁻¹ of N. oculata. The rotifer P. similis was more adversely affected due to the presence of B. ibericus than vice versa. The data are important for developing techniques for a large-scale culture of these rotifers as food in aquaculture.